This proposal deals with translational control of gene expression in mammalian cells which takes a unique position in cellular metabolism. We use a viral system (the picornaviruses) to study the properties and function of two cellular factors, P57 and "initiation correcting factor" (ICF), that we have described previously. Factor p57 has been shown to be essential in the function of IRES (internal ribosomal entry site), a structural element in the 5'non-translation region (5'NTR) of picornavirus mRNA that confers cap-independence of translation. The ICF corrects false initiation of translation of poliovirus RNA in rabbit reticulocytes lysates. The role of the factors in cellular translation is not known. We shall isolate the factors, clone the corresponding cDNAs, determine their sequence, and prepare factor-specific immunological probes. We will purify large amounts by over-expression in suitable cells, and study their biochemical properties, and the effect of possible post-translational modifications with respect to their interaction with picornavirus 5'NTRs. Of particular interest is whether the factors occur unmodified in normal quantities in human neuronal cells, and whether the interaction of either factor with RNA of attenuated polioviruses is modified. These studies will contribute to the molecular understanding of the attenuation phenotype of poliovirus. They will also characterize cellular RNA binding proteins, a class of factors whose interaction with RNA and whose role in cellular metabolism is of central importance yet poorly understood.